1. Field of the Invention
The present invention relates to a powder material of fine particles mainly including a metal such as silicon or tin that is alloyed with lithium by an electrochemical reaction, and a metal oxide or a half metal oxide; an electrode structure including the powder material; and an energy storage device including the electrode structure.
2. Description of the Related Art
Recently, the amount of CO2 gas contained in the atmosphere has been increased, so that the possibility of the green house effect to give rise to global warming has been pointed out. Further, the air pollution due to substances including CO2, NOx, hydrocarbon and the like is causing a serious problem. Moreover, in the light of the increase of crude oil price, attention has been focused on a hybrid vehicle including an engine and an electric motor operated by an electric energy stored in an energy storage device and an electric vehicle from the viewpoint of environmental protection. Therefore, in order to improve the performance of the hybrid vehicle and electric vehicle and reduce the production cost thereof, there is a need for development of a capacitor or an energy storage device such as a secondary battery having both high power density and high energy density.
In addition, the function of portable devices such as a mobile phone, a notebook personal computer, a digital camcorder, a digital camera, a PDA (personal digital assistant) and the like has advanced year by year. As a power source for operation thereof, there is desired the development of a small, lightweight, high-capacity energy storage device such as a secondary battery that can be rapidly charged.
As a representative example of the energy storage device, there is included the so-called “lithium ion battery” of a rocking chair type in which a lithium intercalation compound which deintercalates lithium ions from between layers in a reaction at the time of charging is used as a positive electrode material and a carbon material represented by graphite which intercalates lithium ions between layers of six-membered ring network planes provided by carbon atoms is used as a negative electrode material. The lithium ion battery has a high battery voltage and a high energy density and is therefore in widespread use as a power source for various portable devices and is being considered as a power source for a hybrid vehicle.
However, since the negative electrode is composed of a carbon material, theoretically this “lithium ion battery” allows intercalation of only one sixth in maximum of a lithium atom per one carbon atom of the carbon material. Therefore, further increase in capacity is difficult and a new electrode material for attaining further increase in capacity is being desired. In addition, although the “lithium ion battery” has a high energy density and is expected as a power source for a hybrid vehicle and an electric vehicle, there is a problem that the internal resistance of the battery is too high to release a sufficient amount of electricity at the time of rapid discharge, that is, the power density is small. Therefore, development of an energy storage device with a high power density and a high energy density is desired.
In order to further increase the capacity of the lithium secondary battery including the “lithium ion battery”, the present inventors has proposed a negative electrode for a lithium secondary battery including silicon and tin elements in U.S. Pat. Nos. 6,051,340, 5,795,679, 6,432,585, Japanese Patent Application Laid-Open No. H11-283627, U.S. Pat. Nos. 6,517,974, 6,569,568, Japanese Patent Application Laid-Open No. 2000-311681 and WO 00/17949.
In U.S. Pat. No. 6,051,340 a lithium secondary battery has been proposed which uses a negative electrode formed by providing, on a current collector of a metal material that is not alloyable with lithium, an electrode layer formed of a metal such as silicon and tin that is alloyable with lithium and a metal such as nickel and copper that is not alloyable with lithium.
In U.S. Pat. No. 5,795,679, a negative electrode has been proposed which is formed of powder of an alloy of an element such as nickel and copper and an element such as tin. In U.S. Pat. No. 6,432,585, a lithium secondary battery has been proposed which uses a negative electrode with an electrode material layer containing at least 35 wt. % of particles with an average particle diameter of 0.5 to 60 μm and including silicon, tin, and the like and having a porosity of 0.10 to 0.86 and a density of 1.00 to 6.56 g/cm3.
In Japanese Patent Application Laid-Open No. H11-283627 a lithium secondary battery with a negative electrode including silicon and tin with an amorphous phase. In U.S. Pat. Nos. 6,517,974 and 6,569,568, a lithium secondary battery has been proposed which has as an active material a composite comprised of a positive electrode active material of negative electrode active material with an amorphous phase and a material such as a metal which is electrochemically inactive to substances other than lithium. In Japanese Patent Application Laid-Open No. 2000-311681, a lithium secondary battery has been proposed which uses a negative electrode comprised of particles of an alloy of amorphous tin and a transition metal of a non-stoichiometric composition. In WO 00/17949, a lithium secondary battery has been proposed which uses a negative electrode comprised of particles of an alloy of amorphous silicon and a transition metal of a non-stoichiometric composition.
The lithium secondary battery which uses the above described alloy as a negative electrode can realize a higher capacity and reduce volume expansion at the time of charging. However, also in the above described alloy, lithium is stored in a lattice of silicon or tin at the time of charging to form a lithium alloy. Therefore, the volume expansion is still present and the increase in internal resistance of the battery caused by the repeated charge and discharge is not completely successfully restrained. In addition, since the rate of forming a lithium alloy caused at the time of charging is not so high, it cannot be said that there is no possibility of occurrence of decomposition of an electrolyte solution and deposition of metal lithium onto a negative electrode surface at the time of rapid charging depending on the design of the battery structure. Therefore, there is a need for the development of an energy storage device which maintains high capacity characteristic, has a high power density, and allows rapid charging.
On the other hand, the electric double layer capacitor which uses active carbon with a large specific surface area for a negative electrode and a positive electrode and stores electricity in an electric double layer allows rapid charging and has a large capacity, and is therefore expected as a power source for a hybrid vehicle. The electric double layer capacitor has such a long life as to withstand a charge/discharge frequency about 10 to 100 times greater than the “lithium ion battery” and has a higher power density which is approximately 5 times that of the “lithium ion battery”. On the other hand, the electric double layer capacitor is lower in energy density than the “lithium ion battery”, that is, approximately 1/10 to ½ of that of the “lithium ion battery” in specific energy and approximately 1/50 to ½ of that of the “lithium ion battery” in energy density and has not yet been adopted as a power source for a movable body. Therefore, there is a need for the development of an energy storage device that makes use of the above described characteristics of the electric double layer capacitor, maintaining the advantages of allowing rapid charging, having a long life in repetition use and having a high power density, and has an improved energy density.
In order to solve the problems of the above described electric double layer capacitor, there have been made a proposal of using for an electrode a carbon material which can store/release lithium ions or anions at the time of charging/discharging and a proposal for a hybrid capacitor that uses for an electrode a metal oxide material which can store/release lithium ions at the time of charging/discharging. Such proposals have been made in, for example, Japanese Patent Application Laid-Open Nos. S60-170163, H2-181365, H4-34870, H8-107048, 2000-340470, 2002-25867, 2004-079321, 2005-086113, 2000-268881, 2003-109873, 2003-224037, 2004-103669, and 2004-178828, and Journal of The Electrochemical Society, 148 A930-A939 (2001).
In Japanese Patent Application Laid-Open No. S60-170163, a battery (energy storage device) has been proposed in which a polyacene material which is electrochemically dopable with ions is used for a negative electrode and/or a positive electrode. In Japanese Patent Application Laid-Open No. H2-181365, a capacitor has been proposed in which a polyacene material is used for both a positive and a negative electrodes and a quaternary ammonium salt is used for an electrolyte. In Japanese Patent Application Laid-Open No. H4-34870, a battery (energy storage device) has been proposed in which a polyacene material having lithium carried thereon is used for a negative electrode.
In Japanese Patent Application Laid-Open No. H8-107048, a capacitor has been proposed in which a carbon material having lithium occluded therein is used for a negative electrode and active carbon is used for a positive electrode. In Japanese Patent Application Laid-Open No. 2000-340470, a capacitor has been proposed in which an electrode of a carbon material with micropores containing a metal or a metal compound is used for each of a positive and a negative electrodes. In Japanese Patent Application Laid-Open No. 2002-25867, an electric double layer capacitor has been proposed in which an electrode formed of non-porous carbon including graphite-like microcrystalline carbon having electrolyte ions together with a solvent intercalated between layers is used for each of a positive and a negative electrodes.
In Japanese Patent Application Laid-Open No. 2004-079321, an energy storage device has been proposed in which a composite porous material having a carbon material deposited on an active carbon surface is used for a negative electrode and active carbon is sued for a positive electrode. In Japanese Patent Application Laid-Open No. 2005-086113, an electric double layer capacitor has been proposed which includes an electrode member having pores larger than electrolyte ions obtained by electrically activating a carbon member.
On the other hand, a metal oxide is proposed to be used for an electrode material. In Japanese Patent Application Laid-Open No. 2000-268881, an electrochemical capacitor has been proposed in which an electrode including lithium vanadium oxide and a conductive filler is used for a negative electrode and an electrode including active carbon is used for a positive electrode. In Japanese Patent Application Laid-Open No. 2003-109873, an electric double layer capacitor has been proposed in which a conductive porous ceramic material with a mesoporous structure is used for an electrode. In Japanese Patent Application Laid-Open No. 2003-224037, an electric double layer capacitor has been proposed which uses an electrode obtained by coating a conductive ceramic material on a surface of a porous material.
In Japanese Patent Application Laid-Open No. 2004-103669, a capacitor has been proposed in which a fine carbon powder coated with a metal oxide, a metal nitride or a metal carbide is used as an electrode material. In Japanese Patent Application Laid-Open No. 2004-178828, a nonaqueous lithium energy storage device has been proposed in which a composite porous material having a carbonaceous material deposited on an active carbon surface is used for a negative electrode, an amorphous metal oxide containing at least one of Mn and V is used for a positive electrode and includes a lithium salt as an electrolyte. In Japanese Patent Application Laid-Open No. 2005-108595, an electrode for an electrochemical device has been proposed which contains a tetratitanic acid nanosheet represented by H2Ti8O17.nH2O (n=0−2.0) and a carbon material. In U.S. Pat. No. 6,252,762, a rechargeable energy storage system has been proposed in which a material that reversibly intercalates cations such as alkali metal ions such as Li4Ti5O12 is used for a negative electrode and a material that reversibly adsorbs anions is used for a positive electrode. In addition, in Journal of The Electrochemical Society, 148 A930-A939 (2001), a nonaqueous energy storage cell has been reported which is prepared using a negative electrode including Li4Ti5O12 and a positive electrode including active carbon.
However, any one of the energy storage devices such as a capacitor according to the above described proposals has an energy density which is 1/10 or less of that of a lithium secondary battery (including lithium ion battery), so that further increase in energy density is desired.
Accordingly, there is a continuing need for the development of an energy storage device which has a high energy density close to the energy density of the lithium secondary battery and has a rapid charging performance and a high power density performance for a capacitor.